Data transfer rate in modern communication systems requires a variety of Pulse Amplitude Modulation (PAM) schemes, which assigns multiple power levels to a transmitting symbol, allowing it to represent a plurality of bits. The conventional PAM-4 modulation scheme, for instance, has 4 power levels, each level representing a combination of 2 bits, similarly, a PAM-8 modulation scheme has 8 power levels, each level representing a combination of 3 bits, etc.
FIG. 1 (prior art) illustrates an example of a traditional current mode PAM-4 driver. The differential output voltage levels ON and OP are determined by the input signals, D1 and D0 (both D1 and D0 may take the logic levels “0” or “1”), which control the conductance of the switching transistors M0, M1, M2 and M3, thereby allowing one or more of the two currents I1 and I0, to flow (or not to flow, depending on the combination of input signals D1 and D0) through one or more of the resistors R1 and R0. As a result, different voltage levels are introduced at each output. The resulting voltage level at the negative output ON, for instance, can be calculated by Eq. 1:ON=VDD−[D1P*I0*R0+D0P*I1*R0]  [Eq. 1]
If I1=2I0, the resulting output voltage is given by:
D0PD1PM0M3ON00OFFOFFVDD01OFFONVDD-I0 * R010OFFOFFVDD-2I0 * R011ONOFFVDD-3I0 * R0
However, the traditional PAM-4 driver consumes relatively high power, due to the required multi-power-level operation mode, where current flows through the resistors in 3 out of four states. The main cause for the relative high power dissipation is the use of series current sources I1 and I0, which require high overhead voltage that is deduced from the available voltage swing. Such overhead voltage forces the operation points of the transistors in the circuit to rise for higher output swing, and hence, requires even higher operation voltage VDD. Another cause of this power dissipation is the inability of such PAM-4 drivers to utilize all of the VDD voltage range, as is apparent from Eq. 1 above.
It is therefore an object of the present invention to provide a fast driver which is power efficient for required output swing.
It is another object of the present invention to provide a fast driver which is independent of current source linearity.
It is a further object of the present invention to provide a fast driver which employs small switching transistors with low parasitics.
Other objects and advantages of the invention will become apparent as the description proceeds.